1. Field of the Invention
The present invention relates to an optical pickup apparatus that performs operations for reading signals recorded in optical discs and operations for recording signals into optical discs using a laser beam.
2. Description of the Related Art
Optical disc apparatuses are widely used, each of which is capable of performing a signal reading operation and a signal recording operation with a laser beam emitted from an optical pickup apparatus being applied to a signal recording layer of an optical disc.
Although optical disc apparatuses using optical discs called CD and DVD are generally used, optical disc apparatuses are recently commercialized that use optical discs with record densities improved, i.e., optical discs of the Blu-ray standard.
Infrared light having a wavelength of 785 nm is used as a laser beam for performing the operation of reading signals recorded in a CD-standard optical disc, and red light having a wavelength of 660 nm is used as a laser beam for performing the operation of reading signals recorded in a DVD-standard optical disc.
A transparent protective layer provided between a signal recording layer and an optical disc surface in a CD-standard optical disc has a thickness of 1.2 mm, and a numerical aperture is set at 0.47 in an objective lens used for performing the operation of reading signals from the signal recoding layer. A transparent protective layer provided between a signal recording layer and an optical disc surface in a DVD-standard optical disc has a thickness of 0.6 mm, and a numerical aperture is set at 0.6 in an objective lens used for performing the operation of reading signals from the signal recoding layer.
In contrast to the CD-standard and DVD-standard optical discs, a laser beam having a shorter wave length, for example, blue-violet light having a wavelength of 405 nm is used as a laser beam for performing the operation of reading signals recorded in a Blu-ray-standard optical disc.
A protective layer provided on a top surface of a signal recording layer of the Blu-ray-standard optical disc has a thickness of 0.1 mm, and a numerical aperture is set at 0.85 in an objective lens used for performing the operation of reading signals from the signal recording layer.
It is necessary to reduce a diameter of a laser spot which is formed by a laser beam being focused in order to read signals recorded on the signal recording layer provided in a Blu-ray standard optical disc or to record signals onto the signal recording layer. An objective lens used for acquiring a desired laser spot shape is characterized in not only an increased numerical aperture but also a shortened focal length, and thus a reduced radius of curvature of the objective lens is reduced.
An optical disc apparatus has been commercialized which is capable of performing the operations of reading signals recorded in and recording signals into all the optical discs of the CD standard, the DVD standard, and the Blu-ray standard described above, and an optical pickup apparatus built into such an optical disc apparatus includes a laser diode that emits laser beams having wavelengths corresponding to the above-described standards and an objective lens that focuses the laser beams emitted from the laser diode onto the signal recording layers provided in the optical discs.
The optical pickup apparatus capable of performing the operation of reading signals recorded in the optical discs of all the different standards described above includes two objective lenses, i.e., an objective lens that performs the laser-beam focusing operation for the CD-standard and DVD-standard optical discs and an objective lens that performs the laser-beam focusing operation for the Blu-ray-standard optical disc.
Such an optical pickup apparatus including two objective lenses has not only a problem that a configuration of an optical system becomes complicated but also a problem that a size of the optical pickup apparatus is increased. As a method for solving such problems, such a technique is developed that one objective lens performs the laser-beam focusing operations for the optical discs of all the standards.
A patent document (Japanese Laid-Open Patent Publication No. 2006-172608) discloses an optical pickup apparatus including a semiconductor laser that has light-emitting elements for emitting a first laser beam, a second laser beam, and a third laser beam of different wavelengths housed in the same housing, one objective lens that focuses the first, second, and third laser beams emitted from the semiconductor laser onto three types of optical discs of different standards, and one photodetector irradiated with the return light reflected by an optical disc.
The optical pickup apparatus disclosed in the patent document is configured so as to perform the operation of reading signals recorded in three types of optical discs of different standards using one semiconductor laser, one objective lens, and one photodetector, however, there is such a problem that the apparatus becomes expensive as well as that the configuration thereof becomes complicated since an optical axis correcting element is necessary for correcting misalignment of optical axes.